The present invention relates to a machine-readable hologram containing the record of a parallel bar-shaped pattern, like a bar code pattern, and also to a method of reading such a hologram. More particularly, the present invention relates to a hologram which can be readily read with a single light-receiving element by moving the hologram, and also to a method of reading such a hologram.
It is a known practice to record a bar code in the form of a hologram, reconstruct it by illumination with a laser beam for reconstruction, and read the reconstructed bar code with a line sensor or an image sensor (for example, see Japanese Patent Application Laid-Open (KOKAI) Nos. 62-283383 (1987), 62-283384 (1987), 62-283385 (1987), 63-61386 (1988), 63-168397 (1988), 63-206791 (1988), and 63-207696 (1988).
The prior art, which employs a line sensor or an image sensor to read a recorded bar code, suffers, however, from the problem that there are many restrictions on the installation position of the sensor. If the sensor is not disposed in the required position, particularly in terms of the direction of propagation of the diffracted light, the bar code read accuracy lowers.
In addition, it has not heretofore been practiced to read a recorded bar code with a single light-receiving element having a simple structure.
As another prior art, there is known an information reading method wherein a light beam is moved relative to a diffraction grating pattern formed in a bar code field, and diffracted light from each bar-shaped pattern is read, thereby reading information recorded in the form of a bar code (see U.S. Pat. No. 5,059,776). However, the conventional method involves a restriction on the relationship between the diameter of a light beam used for reading and the widths of bar-shaped patterns recorded. That is, as the spacing of bar code elements becomes smaller, the resolving power for reading degrades. In addition, since the recorded information can be seen directly in the form of a bar code, it is inferior in secrecy to information recorded in the form of a hologram.